1. Field of the Invention
The present invention relates to a method of replacing a portion of the secondary batteries of a battery assembly composed of plural secondary batteries electrically connected in series or in parallel, when some of the secondary batteries are degraded, aged or failed. The present invention especially relates to a method of replacing secondary batteries that are provided in pure electric vehicles (PEV) and hybrid electric vehicles (HEV).
2. Description of the Related Art
Secondary batteries include lead-acid batteries, nickel-cadmium (Nixe2x80x94Cd) batteries, nickel-metal hydride (Nixe2x80x94MH) batteries, lithium ion batteries or the like. After consuming their power, these batteries can be charged by connecting with an external power source and supplying a predetermined current. Because of such characteristics, these batteries have been used in various apparatuses. For example, batteries have been equipped in vehicles to supply power to spark plugs of engines.
Recently, Nixe2x80x94MH batteries are used predominantly due to their high energy density (i.e., energy can be accumulated in a small space) and their high output density to be used as main power sources for driving electric motors in pure electric vehicles (PEV) and/or so-called hybrid electric vehicles (HEV) having both engines and electric motors. In such PEV and HEV, a plurality of cells are assembled in series or in parallel to construct one battery assembly to be used for a battery pack, so that sufficient power can be supplied to motors.
A Nixe2x80x94MH battery formed as an assembly of a plurality of batteries is provided in a PEV or HEV. The battery can be used for a long time under an appropriate operating condition. Generally however, an individual cell of a battery assembly will be out of order or reach the end of its lifetime due to differences between the secondary batteries or failures of parts composing the secondary batteries. The battery assembly cannot function sufficiently even if one secondary battery of the battery assembly is out of order or at the end of its lifetime, and this can result in malfunction of the system. Therefore, a number of methods of detecting abnormality, failures or degradation of secondary batteries have been proposed.
Well-known methods include, for example, a degradation judgment based on a decline of output voltage of a battery, or a degradation judgment based on the specific gravity of an electrolytic solution for a case of a lead-acid battery. JP-A-4-341769 describes a judgment based on a change of voltage-current characteristics caused by a rise of internal resistance of a battery. JP-A-11-89101 describes memorizing battery operation time and judging degradation when the time reaches a predetermined level. In WO 97-27495, degradation is judged by using a temperature rise of a battery.
Conventional techniques concerning replacement of an overall battery assembly are applied to batteries of portable phones, notebook personal computers, video cameras, or the like. A conventional technique concerning partial replacement of a battery assembly is disclosed in JP-A-8-203567. In this description, a terminal voltage at every lead-storage battery composing a battery assembly is observed. When the voltage becomes equal to or lower than a predetermined voltage value, the state is judged as degradation, and a voltage drop display LED lights up.
JP-A-2-101937 describes an example of using a Nixe2x80x94Cd storage battery for a secondary battery. In this description, respective output voltages of a plurality of battery cells are detected sequentially by a voltage detection means. Battery cells having detected output voltages of not more than a standard value are identified and displayed by a displaying means to report the necessity of battery replacement.
However, the above-mentioned conventional techniques concerning partial replacement of a battery assembly do not refer to differences in characteristics between newly added secondary batteries and used batteries of the battery assembly.
In a lead-acid battery, a variation in a state-of-charge (SOC) in the respective batteries after replacement is cancelled by carrying out a constant voltage charge, and the overall battery assembly can be fully charged. However, a Nixe2x80x94MH battery or Nixe2x80x94Cd battery may be overcharged in a constant voltage charge. As a result, the SOC variation between the replacement battery and the initial battery assembly cannot be cancelled, and the batteries cannot deliver the maximum performance.
When a replacement battery has a capacity smaller than an average capacity of the battery assembly, the performance of the original battery assembly cannot be maintained in an intended use where the capacity of the battery assembly is important.
When being applied to a HEV or the like, a secondary battery will be used constantly with the state-of-charge in an intermediate area without being fully charged. In such a case, the following problems can occur. First, a variation in the state-of-charge of the batteries causes a restriction on the discharge side with a battery having a small state-of-charge while the charge side is restricted by a battery having a large state-of-charge. This makes it impossible to make the most use of the battery capacity. Second, a variation in the self-discharge characteristics of the batteries causes a restriction on the discharge side with a battery having a large self-discharge while the charge side is restricted with a battery having small self-discharge. This will cause a variation of the state-of-charge. Third, a variation of the internal resistance of the batteries can hinder sufficient performance of the batteries, since a combination of a replacement battery having a large internal resistance with a battery assembly can cause an error to an actual capacity depending on capacity judgments.
The present invention is made to solve the above-mentioned problems, and has an object of providing a method of replacing secondary batteries at a low cost to provide an overall battery assembly with a maximum performance after the replacement.
Also the present invention provides a method of replacing secondary batteries in which battery characteristics are used as a further standard to judge defective batteries, and a recoverable secondary battery is reused. Accordingly, a cost of replacement batteries is lowered and wastes can be reduced remarkably.
For the above-mentioned purposes, the present invention provides a method of replacing secondary batteries, and the method comprises the steps of:
preparing a battery assembly of a plurality of secondary batteries electrically connected in series or in parallel; and replacing a portion of the plurality of batteries with one or more replacement batteries when the portion is judged as defective as a result of a detection. In the method, voltage is detected for every voltage detection block in order to judge defects of secondary batteries in the voltage detection block unit. Batteries to be replaced, that is, batteries judged as defective, are replaced with replacement batteries in the voltage detection block unit.
According to the method, overall replacement of the battery assembly is not required. Also, there is no need to specify which of the all secondary batteries composing the battery assembly is defective. Since replacement is carried out in a voltage detection block unit to detect and control battery voltage in a conventional construction, detection of defective batteries can be performed only by modifying software without adding any hardware. As a result, the cost of the replacement battery itself and also cost concerning detection of defective batteries can be reduced.
Preferably in the replacing method, a secondary battery having a maximum capacity ranking among a battery group composing the assembly is used for the replacement battery.
Accordingly, the capacity of the battery assembly will not be restricted by the replacement battery, so that the performance of the original battery assembly can be maintained.
It is preferable that when a defect of a battery is detected, secondary batteries positioned in the vicinity of the defective battery to compose a battery assembly also are replaced by replacement batteries. Preferably, an ambient temperature of the batteries to be replaced is detected to determine whether the secondary batteries positioned in the vicinity of the defective battery should be replaced or not.
Accordingly, batteries positioned in the vicinity of a defective battery are replaced before any failure occurs in the battery assembly in the case wherein the temperature is raised locally in the vicinity of the defective batteries or wherein batteries around the defective battery are subjected to a high temperature due to heating of the defective battery itself. In this way, any possible failures of the batteries can be prevented.
Preferably, activated secondary batteries are used for the replacement batteries. As a means to activate secondary batteries, cyclic charge-discharge is preferred. It is particularly preferred to carry out cyclic charge-discharge until the sum of charge-discharge power in the charge-discharge cycle becomes at least 450% of the battery capacity.
FIG. 5 shows a relationship between a mileage of a vehicle and an internal resistance of a battery assembly equipped in the vehicle. As shown in FIG. 5, the internal resistance of the battery assembly is lowered in an initial step of driving, and later it is maintained substantially constant. And the internal resistance is increased at the end of the lifetime of the battery assembly.
FIG. 6 shows a change in a battery internal resistance when a cyclic charge-discharge is carried out for the replacement batteries. In FIG. 6, the initial value is set to be xe2x80x981xe2x80x99. As shown in this drawing, the battery internal resistance fluctuates considerably in the initial state. By performing charge-discharge of approximately 450% or more of the battery capacity, an internal resistance of about 70% to that in the initial state will be obtainable. Therefore, when a cyclic charge-discharge is carried out until the sum of the charge-discharge amount during the cycle is 450% or more of the battery capacity, a difference in the internal resistance between the replacement battery and the other batteries can be corrected even if the internal resistance of the other batteries has changed by the time that the replacement battery is combined in the battery assembly.
Another preferred method to activate a replacement secondary battery is to leave the battery for at least five days while keeping the battery temperature at 50xc2x0 C. or more. By leaving the battery at a temperature of at least 50xc2x0 C. for at least five days, an oxide film on the surface of the activated material inside the battery can be removed efficiently. Accordingly, the internal resistance can be lowered in comparatively cost-effective equipment without conducting a cyclic charge-discharge. When a battery activated by the above means is combined in a battery assembly, a difference between the battery and the other batteries can be corrected, and the battery assembly can deliver sufficient performance.
Preferably, a replacement battery is stored at low temperatures for the above-mentioned purposes.
FIG. 7 shows a state-of-charge versus storage days for a replacement battery by using the temperature as a parameter. As shown in FIG. 7, when stored at high temperatures, the state-of-charge of a battery decreases due to a large self-discharge amount, so the battery cannot be combined immediately in the battery assembly. When the replacement battery is stored at low temperatures, decrease of the state-of-charge due to the self-discharge battery within the battery is controlled. Preferably, the temperatures are not more than 15xc2x0 C., and more preferably, the temperature range is from 0 to 10xc2x0 C. This can prevent battery failures caused by overdischarge when the replacement battery is combined in the battery assembly.
Preferably, a battery assembly is charged to be at least 100% of the battery capacity at a rate of 0.3 C. at most after the replacement of a portion of the secondary batteries in preparation for a subsequent use, since it is possible to cancel a variation in the state-of-charge of the battery assembly and to restrain a significant rise of the battery temperature.
Accordingly, a state-of-charge of the overall battery assembly can be equalized by overcharging the battery assembly including a replacement battery at a low rate in order to cancel differences between the replacement battery and the battery assembly. Furthermore, even when the replaced battery has been left for a long time before the replacement and the internal resistance is increased temporarily, the internal resistance can be recovered to the initial level by the overcharge.
Preferably in the replacement method, defects of the secondary batteries are determined based on a variation range in the charge amount of the battery to be replaced and also a variation range in the self-discharge amount after leaving the batteries for a predetermined period of time.
Accordingly, batteries to be replaced are not disposed of but reused after being treated in a certain manner, when the variation range in the charge amount of the batteries to be replaced is e.g., 2.0 Ah or less, and the variation in the self-discharge amount of the batteries after being left for a predetermined period, e.g., from several weeks to two months, is e.g., 0.5 Ah or less. In this method, a battery to be replaced will not necessarily be replaced by a new battery. Recycling in a material level is available, and this can lower the cost of replacement batteries, and also reduce wastes considerably.